Interesting post, Bob, and generally reflects my experiences
with 8/8/8/8 on 10M and 7/7/7/7 on 15M from the early 1990s
from Colorado.

>Many other times, adding a second or third antenna in phase doesn't increase
>the signal strength of the guys calling me, but it does "smooth out" their
>signals so there is less QSB and more "angle coverage".

But I have one question (or issue). I believe what you are saying is
that the QSB is reduced because of INCREASED angle coverage.
It has been my contention, based on antenna modeling which shows
that the higher angle lobes disappear when using a 4-stack and the
main lobe is compressed to provide the stack gain, AND based on
a lot of experimenting, that the reduction is NOT due to any INCREASE
in angle coverage--but just the opposite. That QSB is due to multipath
--multiple signals arriving at different angles and having differentially
varying phases--causing QSB. By virtue of the STACK you greatly
reduce the amplitude of the higher angle signals so that there is little
out-of-phase signal to cause QSB.

I did some testing with a UA9 a decade or more ago with a single
8L antenna as well as a separate 4-stack. While the 4-stack was
somewhat stronger, the single Yagi was well over S9. On the single
Yagi the UA9 was IMPOSSIBLE to copy due to that "polar distortion"
that was due to multiple arrival angles. But on the stack there was
NO SIGN of the distortion. Conditions were good enough so that we
could work both LP and SP and this removal of distortion was also
confirmed on the LP.

I believe the QSB is due (in many cases) to a couple of angles of
arrival that are slowly varying in phase over time. And that the
polar distortion is also multi-path but with two or more paths that
are varying in phase very rapidly.

In any case, STACKS ROCK! (And I miss them here in south TX--
but they will return one of these days.) And I know the stacks worked
quite well since I had good EME echoes on both 10M and 15M--perfectly
confirming the modeled antenna pattern.